Differentiation is inversely related to development of epithelial tumors. We recently showed that the forkhead transcription factor FOXN1 and the Notch1 signaling pathways are functionally linked to suppression of malignant skin tumor formation. Our working hypothesis is that the retinoid-related nuclear orphan receptor ROR1 is a key integral element of the pro-differentiation network in keratinocytes, functioning as an upstream regulator of FOXN1/Notch1 genes and enzymes involved in lipid metabolism or homeostasis. ROR1 has been previously implicated in a number of processes, including development, inflammation and circadian rhythm. Our preliminary results point to a key role of ROR1 in keratinocyte growth/differentiation and tumor development. For our future work we will address the following specific aims: 1) We will assess whether ROR1 plays an important role in control of keratinocyte differentiation with the FOXN1 and Notch1 genes as downstream effectors. We will test this hypothesis by a combined analysis of primary human keratinocytes (HKCs) in conventional and 3D organotypic cultures, and mice with disruption of the ROR1 gene. 2) We will assess whether ROR1 controls the expression of enzymes involved in lipid metabolism. We will test the possibility that the metabolic function of ROR1 impinges on more general aspects of growth/differentiation control and tumorigenesis via ALOXE3, a lipoxygenase that is intensively involved in keratinocyte differentiation. 3) We will assess whether ROR1 plays an important role in control of keratinocyte tumor formation. This hypothesis will be tested by a dual genetic and pharmacological approach. Mice with heterozygous ROR1 gene mutation will be tested for their sensitivity to DMBA-TPA-induced skin carcinogenesis, and xenografts of HKCs with ras oncogene expression plus/minus ROR1 knockdown will be tested for tumor formation. Pharmacologically, we will test whether treatment with a ROR1 specific agonist (CGP 52608) can affect chemical- induced and/or xenograft tumor formation. )